Thunderbird Configuration Editor Missing – Ask Ubuntu

I use Ubuntu 18.04 LTS and the latest update from Thunderbird.

I get question marks at the end of a sentence when I turn the space. I know that the problem can be fixed by changing "strictly mime" to "true" in the configuration editor. Unfortunately I can not reach the "Config Editor" if I follow the links to "Advanced". It just does not seem. Is there an answer to my dilemma?